Cw Laser Beam Research Articles

Endoscopic double-pulse electronic-speckle-pattern interferometer for technical and medical intracavity inspection

An endoscope electronic-speckle-pattern interferometer (ESPI) camera system is presented that can be applied to examinations of technical objects as well as for in vitro and in vivo minimal invasive medical diagnostics. Integration of optical fibers for the guidance of a cw-laser beam and an endoscopic imaging system yield a compact ESPI system that opens up new possibilities for highly sensitive interferometric intracavity inspection under handheld conditions. A CCD camera in combination with a fast frame-grabber system allows dynamic image subtractions at a frequency rate of as much as 25 Hz with high fringe contrast. Results from investigations of technical objects and biological objects in vitro and in vivo are obtained. In endoscopic minimal invasive therapy this method could substitute for the missing operator's tactile contact with the treated tissue by replacing it with visual information (endoscopic taction).

Complex nonlinear behavior in optically excited nematic liquid crystals.

We present a study of the passage of a monochromatic, linearly polarized cw laser beam through a cell of homeotropically aligned nematic liquid crystal at a slightly oblique angle. The light is polarized perpendicular to the plane of incidence. Experiments in this geometry have revealed the existence of complex, time-dependent dynamics of the director motion. We present a model for the director dynamics derived from the basic equations that shows both periodic and chaotic behavior at different light intensities as observed in the experiments. The model we derive exhibits a variety of complex behaviors, among them an uncommon route to chaos via gluing bifurcations that has not yet been observed in any real physical system.

Diode-pumped Nd:YAG laser producing 122-W CW power at 1.319 /spl mu/m

A high-power diode-pumped Nd:YAG laser oscillating at a wavelength of 1.319 /spl mu/m is reported. A 122-W CW laser beam with an M2 factor of 35 has been achieved with an optical efficiency of 19.6%. The lasing characteristics, including thermal lensing, at 1.319 /spl mu/m are compared with those at 1.06 /spl mu/m. Under lasing conditions, the focal length of thermal lensing at 1.319 /spl mu/m decreases by 25% and increases by 15% at 1.06 /spl mu/m with respect to the nonlasing conditions. Based on the experimental results, the heat dissipation in an Nd:YAG rod is discussed with reference to nonradiative transitions from the upper laser level.

Nonlinear infrared optical measurements of elastic constants in nematic liquid crystal

We present an experimental study of molecular reorientation of nematic liquid crystal director axes induced by a CO 2 cw laser beam. By observation of the Fréederickz Transition Effect, the elastic constants are determined.

Frequency comb generation using femtosecond pulses and cross-phase modulation in optical fiber at arbitrary center frequencies

A technique is presented for generating optical frequency combs centered at arbitrary wavelengths by use of cross-phase modulation (XPM) between a femtosecond pulse train and a cw laser beam by copropagating these signals through an optical fiber. We report results from use of this method to place a 90-MHz frequency comb on an iodine-stabilized Nd:YAG laser at 1064 nm and on a frequency-doubled Nd:YVO(4) laser at 532 nm. XPM is verified to be the comb-generating process, and the width of the frequency comb is measured and compared with theory. The spacing of the frequency comb is compared with the femtosecond source, and a frequency measurement with this comb is demonstrated.

Increase in weld penetration depth by two Nd: YAG laser beams combined

Two multi-kilowatts Nd: YAG laser beams are condensed separately and combined at surface of workpiece in order to increase penetration depth. In this system, one or two laser beams are inevitably inclined depending on size of condensing optics, thus, welding characteristics might be fairly complicated than that of a single laser beam welding. Welding characteristics are evaluated by penetration depth and penetration shape in bead-on-plate tests of stainless steel (304). This paper is intended as an investigation on the welding condition of the deepest weld penetration, especially on arrangement of condensing optics. The deepest weld penetration of 6.3 and 7.7 mm are obtained by two CW laser beams and two rectangularly modulated CW laser beams with total average power of four kilowatts at a welding speed of 5 mm/s, respectively. Deepened penetration depths of weld beads by laser beams in combination due to the increase in total laser power are obtained in comparison to penetration depths of 2.8 and 4.3 mm by a single CW laser beam and a single rectangular modulated CW laser beam with average powers of two kilowatts, respectively. Penetration depths of weld beads obtained with symmetric irradiation exceed those with asymmetric irradiation. The angle between two laser beams for the deepest penetration is 60 degrees for CW laser beams, while the angle is 30 degrees for modulated CW laser beams. The difference in the appropriate angles for the deepest penetration might be explained by different effect of keyhole bottom separation on penetration depths depending on waveform of laser beams. With asymmetric beam arrangement, a deeper penetration is obtained in backhand welding direction for the inclined laser beam than that in forehand direction both in welding using two CW laser beams in combination and two modulated CW beams in combination.

Cavity ringdown spectroscopy with a continuous-wave laser: calculation of coupling efficiency and a new spectrometer design

For the efficient operation of a cavity ringdown spectroscopy (CRDS) system utilized with a continuous-wave (cw) laser, we numerically analyze the coupling efficiency of a cw laser to a ringdown cavity in terms of changes in the scanning rate, the laser linewidth, and the mirror reflectivity. We also demonstrate a new simple design for a CRDS system that can produce a CRDS signal with only a piezoelectric transducer (PZT), without the acousto-optic modulator that is usually adopted to switch off the cw laser beam that enters the cavity. Furthermore, we investigate the feasibility of the cw CRDS technique with a fast-scanning PZT by recording a CRDS spectrum of acetylene overtones. The detection sensitivity that corresponds to the noise-equivalent absorption is found to be approximately 3 x 10(-9)/cm.

Ｎｄ：ＹＡＧレーザ２ビーム合成による溶込み深さの増大

Two multi-kilowatts Nd: YAG laser beams are condensed separately and combined at surface of workpiece in order to increase penetration depth. In this system, one or two laser beams are inevitably inclined depending on size of condensing optics, thus, welding characteristics might be fairly complicated than that of a single laser beam welding. Welding characteristics are evaluated by penetration depth and penetration shape in bead-on-plate tests of stainless steel (304). This paper is intended as an investigation on the welding condition of the deepest weld penetration, especially on arrangement of condensing optics. The deepest weld penetration of 6.3 and 7.7 mm are obtained by two CW laser beams and two rectangularly modulated CW laser beams with total average power of four kilowatts at a welding speed of 5 mm/s, respectively. Deepened penetration depths of weld beads by laser beams in combination due to the increase in total laser power are obtained in comparison to penetration depths of 2.8 and 4.3 mm by a single CW laser beam and a single rectangular modulated CW laser beam with average powers of two kilowatts, respectively. Penetration depths of weld beads obtained with symmetric irradiation exceed those with asymmetric irradiation. The angle between two laser beams for the deepest penetration is 60 degrees for CW laser beams, while the angle is 30 degrees for modulated CW laser beams. The difference in the appropriate angles for the deepest penetration might be explained by different effect of keyhole bottom separation on penetration depths depending on waveform of laser beams. With asymmetric beam arrangement, a deeper penetration is obtained in backhand welding direction for the inclined laser beam than that in forehand direction both in welding using two CW laser beams in combination and two modulated CW beams in combination.

加工点合成Ｎｄ：ＹＡＧレーザ２ビーム溶接におけるキーホール底部分離とその影響因子

Two Nd:YAG laser beams were condensed separately and combined at surface of workpiece in order to increase weld penetration depth. Welding characteristics were evaluated by penetration depth and penetration shape in bead-on-plate tests. Transition from penetration with one bottom to penetration with two bottoms occurs as function of welding condition. Penetration with two bottoms might be a sufficient evidence in favor of keyhole bottom separation. A close study of keyhole bottom separation is necessary to understand the increase of weld penetration by multiple laser beams combined. This paper is intended as an investigation on factors of keyhole bottom separation. The authors found that factors of keyhole bottom separation are angle of incidence of laser beams, laser power density, heat input, distance between two laser beams on a surface of workpiece, and welding position. Keyhole bottom separation occurs under conditions determined by high power density of one laser beam and high total heat input, such as 1500 W/mm2 and 104J/cm, respectively, at angle of incidence of 30° in welding using two CW laser beams in combination.

A Raman Study of a Metastable Crystalline State in Amorphous SiGe Layers Under cw Laser Illumination: Influence of the Temperature

AbstractA metastable crystalline state was observed in amorphous SiGe/Si layers under illumination with a cw laser beam (514.5 nm) with power density below the crystallization threshold. The observation of this state was temperature dependent and it was reversible. The structural properties of this metastable crystallization were studied by Raman spectroscopy.

Factors of keyhole bottom separation in Nd:YAG laser welding with two beams combined at surface of workpiece

Two Nd:YAG laser beams were condensed separately and combined at surface of workpiece in order to increase weld penetration depth. Welding characteristics were evaluated by penetration depth and penetration shape in bead-on-plate tests. Transition from penetration with one bottom to penetration with two bottoms occurs as function of welding condition. Penetration with two bottoms might be a sufficient evidence in favor of keyhole bottom separation. A close study of keyhole bottom separation is necessary to understand the increase of weld penetration by multiple laser beams combined. This paper is intended as an investigation on factors of keyhole bottom separation. The authors found that factors of keyhole bottom separation are angle of incidence of laser beams, laser power density, heat input, distance between two laser beams on a surface of workpiece, and welding position. Keyhole bottom separation occurs under conditions determined by high power density of one laser beam and high total heat input, such as 1500 W/mm2 and 104J/cm, respectively, at angle of incidence of 30° in welding using two CW laser beams in combination.

Measurement of the coherence length of diffusely scattered laser beams with adaptive photodetectors

Original experiments on measurements of the coherence length of diffusely scattered CW laser beams using GaAs adaptive photodetectors based on the photo-EMF effect are presented. Simplicity of the configuration, capability to detect aberrated speckle-like wavefronts with high signal-to-noise ratio, and potentially high speed of operation make this configuration potentially useful for Optical Coherence Tomography and Laser Ranging.

Investigation of IGSCC behavior of sensitized and laser-surface-melted Alloy 600

An attempt was made to modify the surface of sensitized Alloy 600, without affecting the bulk properties, by a laser surface melting (LSM) technique in order to improve its resistance to intergranular stress corrosion cracking (IGSCC) in oxidizing environments. A surface layer of 150–200 μm was melted by a CO 2 CW laser beam. The microstructures of the laser-surface-melted specimens were characterized using optical, scanning, and transmission electron microscopy. Slow strain rate tests (SSRTs) at a strain rate of about 4×10 −7 s −1 were carried out in a 0.1 M sodium tetrathionate (Na 2S 4O 6) solution at room temperature in order to evaluate the resistance to IGSCC of the laser-surface-melted specimens. The microstructure of the melted and resolidified layer had a cellular/columnar structure which consisted of cells 1–2 μm in diameter. The thin, solidified-surface layer was observed to act as an excellent barrier to the initiation and propagation of stress corrosion cracking in the experimental conditions. The fracture mode of the sensitized Alloy 600 changed from a brittle intergranular fracture to a typical ductile transgranular failure. The improved IGSCC resistance of the laser-surface-melted specimens can be attributed in part to Cr redistribution at the boundaries of the cells and grains and in part to the elimination of microstructural inhomogeneities such as precipitates, inclusions at the grain boundaries, during rapid melting and resolidification by the laser surface treatment.

Measurements of negative ion densities by absorption spectroscopy

A novel diagnostic technique for the measurement of negative ions is presented. The absorption of a cw-laser beam with a wavelength close to the maximum of the photodetachment cross section is measured. The laser beam undergoes multiple reflections through the discharge volume using a multipass cell (here a Herriott arrangement). Proof-of-principle measurements of negative hydrogen ions in a hybrid volume source are presented. The measured H− density is in agreement with results of conventional, Langmuir probe-based photodetachment measurements.

Modelling of low power cw laser beam heating effects on A GaAs substrate

A scanning laser beam is a common method used to characterise the optical response of GaAs devices. Laser heating of the substrate, however, can alter the local temperature and hence spuriously shift the values of the electrical parameters of interest. In order to assess the magnitude of this problem, we have solved the steady-state heat equation, with the aid of Kirchhoff's transformation. We show for practical dimensions, that correct temperature prediction does not depend on the lateral boundary conditions. We find that the variable that is most tightly coupled to any temperature increase is the power of the laser beam. Usual approximations for the power dissipation density, in the substrate, were found inadequate. A more complete model that considers power dissipation as an exponential function of substrate depth was found to be necessary. We conclude that for low power applications, i.e. using lasers less than 1 mW, heating effects can be considered negligible. For higher powers our results offer worst-case predictions of the local substrate temperature rise.

A nano-stabilization technique for low repetition rate phase-sensitive optical experiments

A nano-stabilization technique is developed for the observation of Ramsey fringes in low repetition rate Rydberg electron wavepacket experiments. The stabilization system consists of a CCD array monitoring optical interference fringes of a reference CW laser beam passing through a Michelson interferometer, a PID feedback circuit generating an error voltage, and a piezo responding to the error voltage to compensate for geometric fluctuations in the interferometer of nm with a response time ms. The proficiency of the stabilized interferometer is demonstrated in an experiment to observe Ramsey fringes in autoionizing Rydberg states of Xe.

Electrohydrodynamics of Dye Molecules in an Isotropic Liquid Crystal Studied by the Transient Grating Method

The transient grating method using a nanosecond laser pulse as the pump and a CW laser beam as the probe is applied to a dye (methyl red)-doped isotropic liquid crystal (MBBA) under a dc electric field. The diffracted signal decays biexponentially due to the faster (in the μs time scale) heat diffusion and the slower (in the ms time scale) mass diffusion of the dye molecule at zero electric field. No significant changes of the heat diffusion by the electric field are observed. The decay by the mass diffusion is greatly affected by the electric field, but the electrohydrodynamic convective flow is not observed. The complementary grating effect is observed in the mass diffusion under the weak electric field, which is explained well with the hydrodynamic model. The viscosity of the medium and the solute−solvent interactions are affected by the electric field. The activation energy of diffusion, which corresponds to the activation energy of viscosity in the hydrodynamic model, decreases with the electric field. The intermolecular association of methyl red and MBBA increases under the electric field so that the diffusional friction grows. The viscosity decrease by the electric field gives greater effect on the diffusion than the diffusional friction due to intermolecular association. Therefore, the diffusion becomes faster under the electric field.

Manipulation of liquid crystal textures with a focused near infrared laser beam

Optical manipulation of disclinations and defects in liquid crystal films was demonstrated and discussed in terms of mass transfer induced by radiation pressure and of molecular rotation under the optical electric field. Orientation of liquid crystal molecules was controlled by changing the polarization direction of a focused cw laser beam. A disclination line could be deformed by moving the focal spot, just like drawing a bow. A point defect followed the laser beam so that it could be freely transported in the film. When two disclination points were optically manipulated to become fused, the defects disappeared immediately and did not return after switching off the laser.

Picosecond Z-scan study of bound electronic Kerr effect in LiNbO 3 crystal associated with two-photon absorption

crystal have been measured using Z-scan technique with picosecond pulses at 532 nm. The nonlinear absorption coefficient and nonlinear refractive index are determined to be 2.5×10-10 cm/W and 5.3×10-15 cm2/W, respectively. Both sign and magnitude of the measured refractive nonlinearity are considerably different from the reported Z-scan results in LiNbO3 obtained with cw laser beam at 514 nm. The nonlinearities in LiNbO3 induced by 532 nm picosecond pulses are believed to be mainly due to two-photon absorption and bound electronic Kerr effect associated with the two-photon absorption.

Self-Trapping of an Optical Vortex by Use of the Bulk Photovoltaic Effect

We report the first observation of self-trapping of an optical vortex due to the bulk photovoltaic effect. A singly charged vortex nested on a broad cw laser beam is trapped in both transverse dimensions in a lithium niobate crystal. Although the optically induced photovoltaic current is inherently polar, the space charge field gives rise to two-dimensional self-defocusing effects. We observe both circular and elliptical self-trapped vortices, as well as beam deformation due to modulation instability.